As is well known, imperfections or defects (e.g., dislocations, point defects associated with local areas of non-stoichiometry or foreign inclusions) that are present in semiconductor lasers typically have the greatest effect on laser characteristics if the laser is operated at high power level. Among such possible effects are such highly deleterious ones as rapid deterioration of the laser, output power saturation caused by leakage paths, and inhomogeneity of the distribution of the output envelope of the radiation. Thus it is imperative that high power semiconductors lasers, including quantum well lasers, be free of imperfections and defects to the greatest degree possible.
Quantum well lasers are considered to have many advantages over conventional double heterostructure lasers. These advantages include improved high frequency response, narrower linewidths, higher output power, reduced chirp under modulation, lower threshold current densities, and increased temperature coefficient T.sub.o. These advantages have been demonstrated in the GaAs/AlGaAs system.
An advantageous GaAs-based quantum well laser that comprises a graded index separate confinement heterostructure has been reported. See D. Feketa et al., Applied Physics Letters, Vol. 49(24), pages 1659-60. This reference discloses GaAs-based strained layer quantum well lasers, grown by MOCVD, wherein the 4 nm thick Ga.sub.0.63 In.sub.0.37 As quantum well is situated between two 0.2 .mu.m thick Al.sub.x Ga.sub.1-x As regions having continuously graded refractive index, wherein x varies continuously and linearly from 0 to 40%. The devices apparently have powers up to about 30 mW/facet, and emitted at a wavelength of about 1 .mu.m.
As those skilled in the art know, the AlGaAs system has the property that essentially any composition within the system is lattice matched with any other composition within the system. Due to this special property of the AlGaAs system, it is relatively easy to produce a continuously graded index region of the type present in the Feketa et al. lasers.
R. M. Ash, et al., Electronics Letters, Vol. 25(22) pp. 1530-1531 (1988) disclose In P-based graded index separate confinement heterostructure quantum well (GRIN-SCH QW) lasers with continuously graded confinement layers of composition Al.sub.y Ga.sub.0.48-y In.sub.0.52 As, with y varying linearly from 0.35 to 0.25. The GaAlInAs system resembles the AlGaAs system in that in the former the concentration in the growth atmosphere of only the group III species need to be varied. However, other quaternary semiconductor systems do not have this property. The InGaAsP is exemplary of these systems.
On the other hand, the InGaAsP system has pronounced advantages over the InGaAlAs system that would make it highly desirable to be able to use the former in GRIN-SCH QW lasers. In particular, the presence of Al, a strong oxygen getter, makes such highly sensitive and complex devices as GRIN-SCH QW lasers relatively difficult to manufacture, since it requires the vigorous elimination of all sources of oxygen in the growth chamber. Under manufacturing conditions this is a difficult task, as those skilled in the art well know, making GRIN-SCH QW lasers that do not comprise Al-containing semiconductor material preferable to such lasers that comprise such material.
Due at least in part to the above referred to complication in the InGaAsP system, it is generally believed by those skilled in the art that continuously graded index regions are not feasible in this system (and in other multiconstituent systems wherein the lattice constant depends on the composition).
In order to obtain emission wavelengths above about 1.2 .mu.m in InGaAs-based QW lasers, the quantum wells would have to be very thin (e.g., about 2-3 nm). Such thin wells are difficult to make. Furthermore, laser performance might be hampered by the resulting difficulties in the carrier capture process. On the other hand, these relatively long wavelengths are of special interest for optical fiber communication. Of particular recent interest are wavelengths at or near about 1.3 .mu.m or 1.5 .mu.m. For instance, radiation at or near 1.48 .mu.m can serve as pump radiation in a 1.55 .mu.m optical fiber communication system that comprises an Er-doped fiber optical amplifier. To be suitable as a source of such pump radiation a laser should have relatively high output power, since the gain that can be obtained in a given fiber optical amplifier increases with increasing pump power. Exemplarily, such a laser should be capable of providing output power of at least 10 mW/facet, preferably more than 25 or even 50 mW/facet. A. Kasukawa et al., Japanese Journal of Applied Physics, Vol. 28(4), pp. L661-L663, Electronics Letters, Vol. 25(2), pp. 104-105, and Electronics Letters, Vol. 25(10), pp. 659-661 (all incorporated herein by reference) disclose 1.3 .mu.m and 1.5 .mu.m InP-based quantum well lasers that comprise two step-wise graded GaInAsP confinement layers. Although the lasers were reported to have been operated at quite high powers, the prior art devices contain features that potentially can result in defects, and thus could reduce yield and/or lead to decreased lifetime. In particular, the presence of step-wise composition changes in the confinement layers can lead to these and other deleterious results. This is due at least in part to the fact that step-wise compositional change requires the growth of separate layers, and during the pause between layers (when source compositions are changed) it is difficult to prevent defect formation at the interface.
In view of the potential importance of high power, long wavelength quantum well lasers for, e.g., optical amplification in appropriately doped fibers, it would be highly desirable to have available a quantum well laser that emits in the wavelength range above about 1.2 .mu.m (e.g., at a wavelength that is suitable for pumping of a fiber optical amplifier), that can be readily manufactured and that can be operated at relatively high power levels. As was discussed above, high power operation requires that the laser be relatively free of imperfections and defects. Such a desirable laser therefore would comprise design features that tend to reduce the incidence of imperfections and defects. This application discloses such a laser.